The present invention relates generally to pulse width modulation (PWM) control circuits. More particularly, the invention provides methods and apparatus for a PWM fan motor speed control circuit which accepts a pulsed input signal and provides a PWM output signal having an adjustable minimum duty cycle.
Regulated power sources are indispensable in modern electronics. For example, the power supply in a personal computer often needs to receive power input from various outlets. Desktop and laptop computers often have regulated power supplies on the motherboard to supply power to the CPU, memories, and peripheral circuitry. Regulated power supplies are also used in a wide variety of consumer applications, such as home appliances, automobiles, and portable chargers for mobile electronic devices, etc.
Regulated power sources are also used in other applications including controlling the speed of a DC motor, e.g., for driving a cooling fan in a computer. As modern personal computers become more powerful, they consume more power and generate more heat. As a result, regulator circuits for fan motor speed control have become increasingly prevalent.
Pulse Width Modulation (PWM) control circuits have been widely used in switching mode power supplies. In PWM control circuits, the duty cycle of the control pulse is used to control the output of a device. In various applications, it is often desirable to set a minimum duty cycle of the output pulse, such that the duty cycle of the output pulse of the PWM control circuit does not become smaller than a predetermined value.
In particular, single phase brushless electric motors with adjustable speed have been widely researched and applied in such fields as cooling fans, because they provide advantages such as low energy consumption and low noise. For example, Japanese patent publication HEI3-74199 describes a cooling fan system using a temperature sensitive device to convert a temperature measurement to a voltage signal. The voltage signal is compared to a triangle wave to produce pulse width modulated control signals, with a larger duty cycle at high temperatures and a smaller duty cycle at lower temperatures.
U.S. Pat. No. 6,943,517 disclosed a method of using a temperature-sensing device, such as a thermistor, to convert a temperature signal to a voltage signal which is compared with a triangle wave to produce pulse width modulated signal for motor speed control. In addition, U.S. Pat. No. 6,943,517 included minimum duty cycle setting circuit and full speed starting circuit. Motor speed control using a temperature-sensing device is also used in products such as LB11961 from SANYO and TC642 from Microchip.
Even though widely used, conventional techniques for adjusting the speed of single-phase motors have many limitations, such as:                1. In some cases, the control signal for motor speed control is an analog signal provided by a temperature sensing device, and can not be easily adapted for pulsed signal control of motor speed. The pulsed signals may or may not carry temperature related information.        2. Some conventional controllers include a fixed minimum duty cycle setting. In this design, the power consumption tends to be high at high power supply voltages, but may not allow adequate starting power at low power supply voltages.        3. Some conventional controllers accept pulsed control input signal to control the speed of single-phase motors. However, these controllers do not provide a minimum duty cycle. These controllers can not satisfy the requirements of some applications in which a non-zero motor speed is desired when the duty cycle of the input pulsed signal is zero.        
From the above, it is seen that improved PWM control circuits and methods are desired.